During the assembly of complex structures such as aircraft wings where the close fit of various sections is critical in the transfer of load and aerodynamic performance, or important in ensuring excessive build loads are not introduced during assembly, much time and expense is expended understanding, managing and mitigating tolerance build up.
These tolerances can consist of variations in length, thickness, width, angle, and profile all acting in combination. Traditionally these tolerances are analysed in detail during the design phase and a datum structure and allowable range defined. In the case of complex surfaces containing three dimensional curvature such as aircraft wings a point may be reached where datum structures between different parts compete for control of features critical for performance, such as aerodynamic profile, or manufacturability.
Often the cost of further improvements in control over these features has to be balanced against increasingly complex tolerance chains, so it may also be necessary to consider the time needed to assemble such complex structures.
In such cases a convenient way of compensating for various tolerances that still allows the primary performance drivers to meet their requirements will benefit cost, ease of assembly and performance.
Shimming is often used to provide compensation for manufacturing tolerances. However, it is not always acceptable in highly loaded joints for stress or material compatibility reasons. Fettling is also possible but time consuming, and in the case of aircraft structure may require highly skilled operators and machinery if complex parts are to be machined to fit during the assembly phase. The adoption of composites with their inherent limitations in machineability and health and safety limitations on dust creation also provide significant manufacturing challenges.
Significant gains in productivity can be made in areas where complex tolerance interactions can be simply mitigated.